The invention relates to an arrangement for measuring the rotary velocity of a rotatable wheel mounted on a vehicle by means of a sensor formed by thin-film technology. The sensor comprises magnetoresistive sensor elements, and enable measurement of a magnetic field which is modulated by the structure of the wheel in dependence on the rotary velocity thereof.
In such arrangements, which are often used in vehicles for measuring the velocity of a wheel on which the vehicle moves, it is sought to maintain the highest possible insensitivity to electromagnetic radiation coming from the outside, usually referred to as a better EMC behavior. A capacitance is formed between the sensor or the sensor elements thereof on the one hand and the wheel, which is usually made of metal, on the other hand, which capacitance forms a current path for interference currents induced by electromagnetic fields. If this current path is closed, which is usually also the case owing to other capacitances in other places in the vehicle, an interference current will be formed which is superimposed on the signals of the sensor elements and which accordingly interferes with or prevents an evaluation of the sensor signal.
It is an object of the invention to provide develop an arrangement of the kind mentioned in the opening paragraph which is less sensitive to interference currents caused by radiated electromagnetic fields, which may possibly also originate outside the sensor.
According to the invention, this object is achieved in that an electrically conducting layer is provided at the outside of the sensor, which layer reduces the capacitive coupling between the wheel and the sensor elements which would occur without said layer, and which layer in its turn is capacitively coupled to the wheel. The electrically conducting layer, which is made of a metal material, is formed by a thin-film technique at least at one side of the sensor, by deposition thereof on an insulating layer. The conducting layer thickness is approximately 0.2 to 3 xcexcm. in that the electrically conducting layer is electrically contacted outside the substrate on which the sensor elements are built up, the electrical contact being provided without using the thin-film technology. The electrically conducting layer is coupled to a reference potential by means of said electrical contact.
The electrically conducting layer at one side of the sensor has the function of reducing the capacitive coupling between the sensor elements of the sensor on the one hand and the wheel and its toothed structure on the other hand. It is achieved thereby that high-frequency currents induced by electromagnetic fields outside the sensor or the sensor elements flow through the capacitance formed between the electrically conducting layer and the wheel. The capacitance between the sensor elements and the wheel, which is still present as before, is reduced thereby, so that the interference currents flowing across this capacitance are reduced, usually considerably reduced. A new path is thus formed for radiation-induced interference currents by the electrically conducting layer, which path guides the interference currents away from the sensor elements.
The layer must be made of metal and be electrically conducting if it is to form the capacitance as effectively as possible. Advantageously, according to the invention, it is formed by thin-film technology, so that it can be manufactured during the same manufacturing process as the sensor elements. The electrically conducting layer is manufactured on an insulation layer, so that complete insulation between the electrically conducting layer and the sensor elements of the sensor is safeguarded. The electrically conducting layer has a layer thickness of approximately 0.2 to 3 xcexcm, so that it is of itself so thin that electromagnetic fields acting on the electrically conducting layer from the outside do not form interference currents in this layer itself, which would be undesirable.
According to a further characteristic of the invention, the electrically conducting layer is electrically contacted exclusively outside the substrate on which the sensor elements are manufactured. This has the advantage in particular that it can be tested whether complete electrical insulation has been obtained between the layers in which the sensor elements are formed and the electrically conducting layer after the manufacture of the sensor elements, the insulation layer, and the electrically conducting layer, but before the electrically conducting layer is contacted. This can be measured first. Only then, advantageously, can the electrically conducting layer be electrically contacted outside the substrate. This electrical contact is achieved by means which do not form part of thin-film technology, i.e. the other than those used for thin-film manufacture of the layers of the sensor elements, the insulating layer, and the electrically conducting layer.
The electrically conducting layer is coupled via an electrical contact to a reference potential which corresponds to the reference potential of the sensor, so as to create a capacitive coupling between the electrically conducting layer and the wheel in an optimum manner.
It is achieved with this arrangement that high-frequency interference currents caused by radiation from electromagnetic fields not only in the sensor, but also, for example, in its supply lines preferably flow through the capacitance formed between the electrically conducting layer and the wheel of the vehicle. Interference currents across the sensor elements are thereby reduced, or possibly substantially eliminated, so that the currents supplied by the sensor elements which serve to measure the rotational velocity are considerably less affected by HF interference currents.
Furthermore, the construction of the electrically conducting layer is achieved in a simple manner because thin-film technology is used for manufacturing this layer and also the subjacent insulation layer, using the same means as far the manufacture of the sensor and the sensor elements thereof.
A clearly reduced sensitivity of the sensor to electromagnetic interference radiation is achieved overall, while the additional expenditure required for this is small.
If the arrangement for measuring rotational velocity is used for an anti-blocking system, as in an embodiment of the invention; the reduced EMC is of particular importance in that the arrangement of the sensor in the wheelbox is particularly sensitive to electromagnetic radiation, while these sensors have supply lines which are also laid in particularly interference-sensitive regions of the vehicle.
The electrical contacting of the electrically conducting by contacts provided outside the substrate of the sensor elements may be advantageously achieved by means of bonding, as in a further embodiment of the invention. This is advantageous also because the electrical connections of the sensor elements to the sensor are usually also made by bonding. An electrical contact for bonding the electrically conducting layer can thus be manufactured in the same process step, so that this involves hardly any additional expenditure.
In a further embodiment of the invention, the electrically conducting layer advantageously covers the side of the sensor facing towards the wheel. The electrically conducting layer is capable of forming the desired capacitance between itself and the wheel and thus reducing the capacitance between the sensor and the wheel especially at this side.
The electrical signals supplied by the sensor elements are normally evaluated in a first stage by an evaluation circuit which is arranged in the immediate vicinity of the sensor elements. As provided for in a further embodiment of the invention, the electrically conducting layer may advantageously also cover this evaluation circuit at one side and thus improve the EMC further in that the path to the capacitance to the wheel is provided already in the region of the evaluation electronics.
The electrically conducting layer need not be a fully closed layer, instead, for example, it may have a comb structure for a further embodiment of the invention. Alternative structures are also conceivable as long as the object mentioned above is achieved.
Advantageously, the sensor comprises four magnetoresistive sensor elements forming a bridge. That provides a particularly good measurement and has found wide acceptance, in example, for ABS sensor applications.